I think you're missing what really went wrong here.
The thing that soured this was NOT that you had been with someone else before. Really, I can promise that: two people can have sex for the first time as a couple and have it be special even when they're in their 60's.
What sullied things was that you were DISHONEST.
What I'd suggest in seeking for resolution with this is that you don't lose sight of that and work with THAT, both alone and in your relationship, rather than making this about who you did have intercourse with for the first time.
Waiting one more month may or may not have made things any different. telling the truth is what would have made a difference here.